1. Field of the Invention
This invention relates to a medium-power battery of fuel cells, which can be used as autonomous electricity generator or complementary power supply system for an individual household.
2. Description of the Related Art
High-temperature fuel cells achieve high efficiency in the process of converting chemical energy into electricity. One advantage of the converters is their quiet operation without mechanical vibrations and a favorable relationship of the unit weight and the dimensions to the electric power generated. It is expected that electrochemical converters will become supplementary parts of electricity generators which draw energy from renewable energy sources which are unable to provide continuous power output over a full 24-hour cycle, such as solar cells or wind turbines.
A typical high-temperature fuel cell contains solid electrolyte, which is usually based on yttrium-stabilized zirconia. On the opposite sides of the thin electrolyte layer are located the anode and the cathode. The electrodes are transparent to gases.
At high temperature, zirconia ceramics have strong ionic conductivity. Due to the difference in partial pressure of oxygen on both sides of the electrolyte, the electrolyte becomes polarized. The partial pressure difference is due to the fact that zirconia ceramics is permeable to oxygen ions only that are produced in the cathode layer during the decomposition of molecular oxygen and taking up free electrons from the cathode. Thus, the cathode is positively polarized, whereas on the anode side free electrons are liberated due to the reaction of the oxygen ions with fuel—hydrogen or carbon oxide—which, in turn, leads to negative polarization. Connecting an external electrical circuit to the electrodes will cause electric current to flow and balance the internal ionic current.
From Polish Pat. No. 213349, a fuel cell is known with a flat laminar ceramic substrate, the core of which is made of a central ceramic plate with high density and rigidity. The ceramic plate is permanently joined with porous cermet layers with distribution channels formed therein. The ceramic substrate so prepared has, on both sides on a part of the surface, layers of solid electrolyte applied and durably bonded to the substrate, which in turn, on a part of their surface, are covered with electrode layers. Such layers of solid electrolyte have their surfaces covered with contact layers. The description presents also applications of the cell in a converter for converting electrochemical to electrical energy, in which the battery of fuel cells is covered with foamed ceramic thermal insulation.
From U.S. Pat. Nos. 4,276,355 and 7,531,053, plate chemical to electrical energy converters are known, which contain multiple electrically connected fuel cells. The fuel cells are configured in a stack, whereby each ceramic structure of the cell is located between two plates. Each of the plates constitutes a separator between the adjacent ceramic structures and has, on both sides, open longitudinal channels for the passage of fuel and air. The fuel channels are arranged orthogonally in relation to the air flow channels. The separators referred to in U.S. Pat. No. 7,531,053 are made of stainless steel with linear thermal expansion coefficient close to that of the electrolyte.
One advantage of the plate converters is the relatively high value of electric power generated per volume of the unit of up to 5 kW/L. This enables the use of an insulated chamber having compact dimensions, which translates into relatively low heat loss, since the amount of distributed heat is proportional to the chamber area. However, the compact design of the batteries also makes it impossible for the batteries to be warmed up and cooled down. This parameter is particularly important in medium and high-power equipment because it may take up to 24 hours for a safe warm-up. With such a long start up time, the batteries of cells cannot serve as emergency or supplementary equipment during peak power consumption. Clearly much opportunity of improvement remains in this technological area.